Folding revamp.

This commit makes type folding more like the way chalk does it.

Currently, `TypeFoldable` has `fold_with` and `super_fold_with` methods.
- `fold_with` is the standard entry point, and defaults to calling
  `super_fold_with`.
- `super_fold_with` does the actual work of traversing a type.
- For a few types of interest (`Ty`, `Region`, etc.) `fold_with` instead
  calls into a `TypeFolder`, which can then call back into
  `super_fold_with`.

With the new approach, `TypeFoldable` has `fold_with` and
`TypeSuperFoldable` has `super_fold_with`.
- `fold_with` is still the standard entry point, *and* it does the
  actual work of traversing a type, for all types except types of
  interest.
- `super_fold_with` is only implemented for the types of interest.

Benefits of the new model.
- I find it easier to understand. The distinction between types of
  interest and other types is clearer, and `super_fold_with` doesn't
  exist for most types.
- With the current model is easy to get confused and implement a
  `super_fold_with` method that should be left defaulted. (Some of the
  precursor commits fixed such cases.)
- With the current model it's easy to call `super_fold_with` within
  `TypeFolder` impls where `fold_with` should be called. The new
  approach makes this mistake impossible, and this commit fixes a number
  of such cases.
- It's potentially faster, because it avoids the `fold_with` ->
  `super_fold_with` call in all cases except types of interest. A lot of
  the time the compile would inline those away, but not necessarily
  always.

compiler/rustc_const_eval/src/interpret/util.rs

@@ -1,5 +1,5 @@
 use rustc_middle::mir::interpret::InterpResult;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use std::convert::TryInto;
 use std::ops::ControlFlow;
 
@@ -47,7 +47,7 @@ fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                         match (is_used, subst.needs_subst()) {
                             // Just in case there are closures or generators within this subst,
                             // recurse.
-                            (true, true) => return subst.super_visit_with(self),
+                            (true, true) => return subst.visit_with(self),
                             // Confirm that polymorphization replaced the parameter with
                             // `ty::Param`/`ty::ConstKind::Param`.
                             (false, true) if cfg!(debug_assertions) => match subst.unpack() {

compiler/rustc_infer/src/infer/canonical/canonicalizer.rs

@@ -11,7 +11,7 @@
 };
 use crate::infer::InferCtxt;
 use rustc_middle::ty::flags::FlagComputation;
-use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::GenericArg;
 use rustc_middle::ty::{self, BoundVar, InferConst, List, Ty, TyCtxt, TypeFlags};
 use std::sync::atomic::Ordering;

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -70,7 +70,7 @@
     self,
     error::TypeError,
     subst::{GenericArgKind, Subst, SubstsRef},
-    Binder, EarlyBinder, List, Region, Ty, TyCtxt, TypeFoldable,
+    Binder, EarlyBinder, List, Region, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
 };
 use rustc_span::{sym, symbol::kw, BytePos, DesugaringKind, Pos, Span};
 use rustc_target::spec::abi;

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -10,7 +10,7 @@
 use rustc_hir::intravisit::{walk_ty, Visitor};
 use rustc_hir::{self as hir, GenericBound, Item, ItemKind, Lifetime, LifetimeName, Node, TyKind};
 use rustc_middle::ty::{
-    self, AssocItemContainer, StaticLifetimeVisitor, Ty, TyCtxt, TypeFoldable, TypeVisitor,
+    self, AssocItemContainer, StaticLifetimeVisitor, Ty, TyCtxt, TypeSuperFoldable, TypeVisitor,
 };
 use rustc_span::symbol::Ident;
 use rustc_span::Span;

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -11,7 +11,7 @@
 use rustc_hir::intravisit::Visitor;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::print::RegionHighlightMode;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperFoldable, TypeVisitor};
 use rustc_span::{Span, Symbol};
 
 use std::ops::ControlFlow;

compiler/rustc_infer/src/infer/freshen.rs

@@ -34,7 +34,7 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::infer::unify_key::ToType;
 use rustc_middle::ty::fold::TypeFolder;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
 use std::collections::hash_map::Entry;
 
 pub struct TypeFreshener<'a, 'tcx> {

compiler/rustc_infer/src/infer/fudge.rs

@@ -1,4 +1,4 @@
-use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{self, ConstVid, FloatVid, IntVid, RegionVid, Ty, TyCtxt, TyVid};
 
 use super::type_variable::TypeVariableOrigin;

compiler/rustc_infer/src/infer/mod.rs

@@ -23,7 +23,7 @@
 use rustc_middle::mir::interpret::{ErrorHandled, EvalToConstValueResult};
 use rustc_middle::traits::select;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::relate::RelateResult;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, SubstsRef};
 pub use rustc_middle::ty::IntVarValue;

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -27,7 +27,7 @@
 use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::ty::error::TypeError;
-use rustc_middle::ty::fold::{TypeFoldable, TypeVisitor};
+use rustc_middle::ty::fold::{TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
 use rustc_span::Span;

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -9,7 +9,7 @@
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::subst::{GenericArgKind, Subst};
 use rustc_middle::ty::{
-    self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeVisitor,
+    self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor,
 };
 use rustc_span::Span;
 

compiler/rustc_infer/src/infer/resolve.rs

@@ -1,7 +1,7 @@
 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use super::{FixupError, FixupResult, InferCtxt, Span};
 use rustc_middle::mir;
-use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFolder, TypeVisitor};
+use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFolder, TypeSuperFoldable, TypeVisitor};
 use rustc_middle::ty::{self, Const, InferConst, Ty, TyCtxt, TypeFoldable};
 
 use std::ops::ControlFlow;

compiler/rustc_infer/src/traits/structural_impls.rs

@@ -60,10 +60,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 // TypeFoldable implementations.
 
 impl<'tcx, O: TypeFoldable<'tcx>> TypeFoldable<'tcx> for traits::Obligation<'tcx, O> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(traits::Obligation {
             cause: self.cause,
             recursion_depth: self.recursion_depth,
@@ -72,7 +69,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.predicate.visit_with(visitor)?;
         self.param_env.visit_with(visitor)
     }

compiler/rustc_lint/src/types.rs

@@ -8,7 +8,7 @@
 use rustc_hir::{is_range_literal, Expr, ExprKind, Node};
 use rustc_middle::ty::layout::{IntegerExt, LayoutOf, SizeSkeleton};
 use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::{self, AdtKind, DefIdTree, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, AdtKind, DefIdTree, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
 use rustc_span::source_map;
 use rustc_span::symbol::sym;
 use rustc_span::{Span, Symbol, DUMMY_SP};

compiler/rustc_macros/src/type_foldable.rs

@@ -30,14 +30,14 @@ pub fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::
     s.bound_impl(
         quote!(::rustc_middle::ty::fold::TypeFoldable<'tcx>),
         quote! {
-            fn try_super_fold_with<__F: ::rustc_middle::ty::fold::FallibleTypeFolder<'tcx>>(
+            fn try_fold_with<__F: ::rustc_middle::ty::fold::FallibleTypeFolder<'tcx>>(
                 self,
                 __folder: &mut __F
             ) -> Result<Self, __F::Error> {
                 Ok(match self { #body_fold })
             }
 
-            fn super_visit_with<__F: ::rustc_middle::ty::fold::TypeVisitor<'tcx>>(
+            fn visit_with<__F: ::rustc_middle::ty::fold::TypeVisitor<'tcx>>(
                 &self,
                 __folder: &mut __F
             ) -> ::std::ops::ControlFlow<__F::BreakTy> {

compiler/rustc_middle/src/macros.rs

@@ -52,14 +52,14 @@ macro_rules! TrivialTypeFoldableImpls {
     (for <$tcx:lifetime> { $($ty:ty,)+ }) => {
         $(
             impl<$tcx> $crate::ty::fold::TypeFoldable<$tcx> for $ty {
-                fn try_super_fold_with<F: $crate::ty::fold::FallibleTypeFolder<$tcx>>(
+                fn try_fold_with<F: $crate::ty::fold::FallibleTypeFolder<$tcx>>(
                     self,
                     _: &mut F
                 ) -> ::std::result::Result<$ty, F::Error> {
                     Ok(self)
                 }
 
-                fn super_visit_with<F: $crate::ty::fold::TypeVisitor<$tcx>>(
+                fn visit_with<F: $crate::ty::fold::TypeVisitor<$tcx>>(
                     &self,
                     _: &mut F)
                     -> ::std::ops::ControlFlow<F::BreakTy>
@@ -95,14 +95,14 @@ macro_rules! EnumTypeFoldableImpl {
         impl<$($p),*> $crate::ty::fold::TypeFoldable<$tcx> for $s
             $(where $($wc)*)*
         {
-            fn try_super_fold_with<V: $crate::ty::fold::FallibleTypeFolder<$tcx>>(
+            fn try_fold_with<V: $crate::ty::fold::FallibleTypeFolder<$tcx>>(
                 self,
                 folder: &mut V,
             ) -> ::std::result::Result<Self, V::Error> {
                 EnumTypeFoldableImpl!(@FoldVariants(self, folder) input($($variants)*) output())
             }
 
-            fn super_visit_with<V: $crate::ty::fold::TypeVisitor<$tcx>>(
+            fn visit_with<V: $crate::ty::fold::TypeVisitor<$tcx>>(
                 &self,
                 visitor: &mut V,
             ) -> ::std::ops::ControlFlow<V::BreakTy> {

compiler/rustc_middle/src/mir/mod.rs

@@ -7,7 +7,7 @@
 use crate::mir::visit::MirVisitable;
 use crate::ty::adjustment::PointerCast;
 use crate::ty::codec::{TyDecoder, TyEncoder};
-use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeVisitor};
+use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::subst::{GenericArg, InternalSubsts, Subst, SubstsRef};
 use crate::ty::{self, List, Ty, TyCtxt};
@@ -3399,20 +3399,14 @@ pub(crate) fn variant(
 TrivialTypeFoldableAndLiftImpls! { ProjectionKind, }
 
 impl<'tcx> TypeFoldable<'tcx> for UserTypeProjection {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(UserTypeProjection {
             base: self.base.try_fold_with(folder)?,
             projs: self.projs.try_fold_with(folder)?,
         })
     }
 
-    fn super_visit_with<Vs: TypeVisitor<'tcx>>(
-        &self,
-        visitor: &mut Vs,
-    ) -> ControlFlow<Vs::BreakTy> {
+    fn visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> ControlFlow<Vs::BreakTy> {
         self.base.visit_with(visitor)
         // Note: there's nothing in `self.proj` to visit.
     }

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -16,10 +16,7 @@
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         use crate::mir::TerminatorKind::*;
 
         let kind = match self.kind {
@@ -93,7 +90,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         Ok(Terminator { source_info: self.source_info, kind })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         use crate::mir::TerminatorKind::*;
 
         match self.kind {
@@ -144,50 +141,41 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
 }
 
 impl<'tcx> TypeFoldable<'tcx> for GeneratorKind {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Place<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(Place {
             local: self.local.try_fold_with(folder)?,
             projection: self.projection.try_fold_with(folder)?,
         })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.local.visit_with(visitor)?;
         self.projection.visit_with(visitor)
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<PlaceElem<'tcx>> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_place_elems(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Rvalue<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         use crate::mir::Rvalue::*;
         Ok(match self {
             Use(op) => Use(op.try_fold_with(folder)?),
@@ -237,7 +225,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         use crate::mir::Rvalue::*;
         match *self {
             Use(ref op) => op.visit_with(visitor),
@@ -288,10 +276,7 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Operand<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(match self {
             Operand::Copy(place) => Operand::Copy(place.try_fold_with(folder)?),
             Operand::Move(place) => Operand::Move(place.try_fold_with(folder)?),
@@ -299,7 +284,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match *self {
             Operand::Copy(ref place) | Operand::Move(ref place) => place.visit_with(visitor),
             Operand::Constant(ref c) => c.visit_with(visitor),
@@ -308,10 +293,7 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
 }
 
 impl<'tcx> TypeFoldable<'tcx> for PlaceElem<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         use crate::mir::ProjectionElem::*;
 
         Ok(match self {
@@ -326,10 +308,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn super_visit_with<Vs: TypeVisitor<'tcx>>(
-        &self,
-        visitor: &mut Vs,
-    ) -> ControlFlow<Vs::BreakTy> {
+    fn visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> ControlFlow<Vs::BreakTy> {
         use crate::mir::ProjectionElem::*;
 
         match self {
@@ -341,44 +320,41 @@ fn super_visit_with<Vs: TypeVisitor<'tcx>>(
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Field {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for GeneratorSavedLocal {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
 
 impl<'tcx, R: Idx, C: Idx> TypeFoldable<'tcx> for BitMatrix<R, C> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Constant<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(Constant {
             span: self.span,
             user_ty: self.user_ty.try_fold_with(folder)?,
             literal: self.literal.try_fold_with(folder)?,
         })
     }
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.literal.visit_with(visitor)?;
         self.user_ty.visit_with(visitor)
     }
@@ -390,6 +366,12 @@ fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Se
         folder.try_fold_mir_const(self)
     }
 
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_mir_const(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ConstantKind<'tcx> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
@@ -406,8 +388,4 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
             ConstantKind::Val(_, t) => t.visit_with(visitor),
         }
     }
-
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        visitor.visit_mir_const(*self)
-    }
 }

compiler/rustc_middle/src/ty/erase_regions.rs

@@ -1,5 +1,5 @@
 use crate::mir;
-use crate::ty::fold::{TypeFoldable, TypeFolder};
+use crate::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use crate::ty::{self, Ty, TyCtxt, TypeFlags};
 
 pub(super) fn provide(providers: &mut ty::query::Providers) {

compiler/rustc_middle/src/ty/fold.rs

@@ -15,16 +15,20 @@
 //! the ones containing the most important type-related information, such as
 //! `Ty`, `Predicate`, `Region`, and `Const`.
 //!
-//! There are two traits involved in each traversal type.
-//! - The first trait is `TypeFoldable`, which is implemented once for many
-//!   types. This includes both (a) types of interest, and (b) all other
-//!   relevant types, including generic containers like `Vec` and `Option`. It
-//!   defines a "skeleton" of how they should be traversed, for both folding
-//!   and visiting.
-//! - The second trait is `TypeFolder`/`FallibleTypeFolder` (for
-//!   infallible/fallible folding traversals) or `TypeVisitor` (for visiting
-//!   traversals). One of these is implemented for each folder/visitor. This
-//!   defines how types of interest are handled.
+//! There are three traits involved in each traversal type.
+//! - `TypeFoldable`. This is implemented once for many types. This includes
+//!   both:
+//!   - Types of interest, for which the the methods delegate to the
+//!     folder/visitor.
+//!   - All other types, including generic containers like `Vec` and `Option`.
+//!     It defines a "skeleton" of how they should be traversed, for both
+//!     folding and visiting.
+//! - `TypeSuperFoldable`. This is implemented only for each type of interest,
+//!   and defines the traversal "skeleton" for these types.
+//! - `TypeFolder`/`FallibleTypeFolder` (for infallible/fallible folding
+//!   traversals) or `TypeVisitor` (for visiting traversals). One of these is
+//!   implemented for each folder/visitor. This defines how types of interest
+//!   are folded/visited.
 //!
 //! This means each traversal is a mixture of (a) generic traversal operations,
 //! and (b) custom fold/visit operations that are specific to the
@@ -32,22 +36,23 @@
 //! - The `TypeFoldable` impls handle most of the traversal, and call into
 //!   `TypeFolder`/`FallibleTypeFolder`/`TypeVisitor` when they encounter a
 //!   type of interest.
-//! - A `TypeFolder`/`FallibleTypeFolder`/`TypeVisitor` may also call back into
-//!   a `TypeFoldable` impl, because (a) the types of interest are recursive
-//!   and can contain other types of interest, and (b) each folder/visitor
-//!   might provide custom handling only for some types of interest, or only
-//!   for some variants of each type of interest, and then use default
-//!   traversal for the remaining cases.
+//! - A `TypeFolder`/`FallibleTypeFolder`/`TypeVisitor` may call into another
+//!   `TypeFoldable` impl, because some of the types of interest are recursive
+//!   and can contain other types of interest.
+//! - A `TypeFolder`/`FallibleTypeFolder`/`TypeVisitor` may also call into
+//!   a `TypeSuperFoldable` impl, because each folder/visitor might provide
+//!   custom handling only for some types of interest, or only for some
+//!   variants of each type of interest, and then use default traversal for the
+//!   remaining cases.
 //!
 //! For example, if you have `struct S(Ty, U)` where `S: TypeFoldable` and `U:
-//! TypeFoldable`, and an instance `S(ty, u)`, it would be visited like so:
+//! TypeFoldable`, and an instance `s = S(ty, u)`, it would be visited like so:
 //! ```text
 //! s.visit_with(visitor) calls
-//! - s.super_visit_with(visitor) calls
-//!   - ty.visit_with(visitor) calls
-//!     - visitor.visit_ty(ty) may call
-//!       - ty.super_visit_with(visitor)
-//!   - u.visit_with(visitor)
+//! - ty.visit_with(visitor) calls
+//!   - visitor.visit_ty(ty) may call
+//!     - ty.super_visit_with(visitor)
+//! - u.visit_with(visitor)
 //! ```
 use crate::mir;
 use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
@@ -66,18 +71,17 @@
 /// To implement this conveniently, use the derive macro located in
 /// `rustc_macros`.
 pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
-    /// The main entry point for folding. To fold a value `t` with a folder `f`
+    /// The entry point for folding. To fold a value `t` with a folder `f`
     /// call: `t.try_fold_with(f)`.
     ///
-    /// For types of interest (such as `Ty`), this default is overridden with a
-    /// method that calls a folder method specifically for that type (such as
+    /// For most types, this just traverses the value, calling `try_fold_with`
+    /// on each field/element.
+    ///
+    /// For types of interest (such as `Ty`), the implementation of method
+    /// calls a folder method specifically for that type (such as
     /// `F::try_fold_ty`). This is where control transfers from `TypeFoldable`
     /// to `TypeFolder`.
-    ///
-    /// For other types, this default is used.
-    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
-        self.try_super_fold_with(folder)
-    }
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error>;
 
     /// A convenient alternative to `try_fold_with` for use with infallible
     /// folders. Do not override this method, to ensure coherence with
@@ -86,40 +90,17 @@ fn fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
         self.try_fold_with(folder).into_ok()
     }
 
-    /// Traverses the type in question, typically by calling `try_fold_with` on
-    /// each field/element. This is true even for types of interest such as
-    /// `Ty`. This should only be called within `TypeFolder` methods, when
-    /// non-custom traversals are desired for types of interest.
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error>;
-
-    /// A convenient alternative to `try_super_fold_with` for use with
-    /// infallible folders. Do not override this method, to ensure coherence
-    /// with `try_super_fold_with`.
-    fn super_fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
-        self.try_super_fold_with(folder).into_ok()
-    }
-
     /// The entry point for visiting. To visit a value `t` with a visitor `v`
     /// call: `t.visit_with(v)`.
     ///
-    /// For types of interest (such as `Ty`), this default is overridden with a
-    /// method that calls a visitor method specifically for that type (such as
+    /// For most types, this just traverses the value, calling `visit_with` on
+    /// each field/element.
+    ///
+    /// For types of interest (such as `Ty`), the implementation of this method
+    /// that calls a visitor method specifically for that type (such as
     /// `V::visit_ty`). This is where control transfers from `TypeFoldable` to
     /// `TypeVisitor`.
-    ///
-    /// For other types, this default is used.
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        self.super_visit_with(visitor)
-    }
-
-    /// Traverses the type in question, typically by calling `visit_with` on
-    /// each field/element. This is true even for types of interest such as
-    /// `Ty`. This should only be called within `TypeVisitor` methods, when
-    /// non-custom traversals are desired for types of interest.
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
 
     /// Returns `true` if `self` has any late-bound regions that are either
     /// bound by `binder` or bound by some binder outside of `binder`.
@@ -219,9 +200,40 @@ fn still_further_specializable(&self) -> bool {
     }
 }
 
+// This trait is implemented for types of interest.
+pub trait TypeSuperFoldable<'tcx>: TypeFoldable<'tcx> {
+    /// Provides a default fold for a type of interest. This should only be
+    /// called within `TypeFolder` methods, when a non-custom traversal is
+    /// desired for the value of the type of interest passed to that method.
+    /// For example, in `MyFolder::try_fold_ty(ty)`, it is valid to call
+    /// `ty.try_super_fold_with(self)`, but any other folding should be done
+    /// with `xyz.try_fold_with(self)`.
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error>;
+
+    /// A convenient alternative to `try_super_fold_with` for use with
+    /// infallible folders. Do not override this method, to ensure coherence
+    /// with `try_super_fold_with`.
+    fn super_fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self {
+        self.try_super_fold_with(folder).into_ok()
+    }
+
+    /// Provides a default visit for a type of interest. This should only be
+    /// called within `TypeVisitor` methods, when a non-custom traversal is
+    /// desired for the value of the type of interest passed to that method.
+    /// For example, in `MyVisitor::visit_ty(ty)`, it is valid to call
+    /// `ty.super_visit_with(self)`, but any other visiting should be done
+    /// with `xyz.visit_with(self)`.
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
+}
+
 /// This trait is implemented for every folding traversal. There is a fold
 /// method defined for every type of interest. Each such method has a default
-/// that does an "identity" fold.
+/// that does an "identity" fold. Implementations of these methods often fall
+/// back to a `super_fold_with` method if the primary argument doesn't
+/// satisfy a particular condition.
 ///
 /// If this folder is fallible (and therefore its [`Error`][`TypeFolder::Error`]
 /// associated type is something other than the default `!`) then

compiler/rustc_middle/src/ty/instance.rs

@@ -1,7 +1,7 @@
 use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::subst::{InternalSubsts, Subst};
-use crate::ty::{self, EarlyBinder, SubstsRef, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{self, EarlyBinder, SubstsRef, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::Namespace;
 use rustc_hir::def_id::{CrateNum, DefId};

compiler/rustc_middle/src/ty/mod.rs

@@ -9,7 +9,9 @@
 //!
 //! ["The `ty` module: representing types"]: https://rustc-dev-guide.rust-lang.org/ty.html
 
-pub use self::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeVisitor};
+pub use self::fold::{
+    FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitor,
+};
 pub use self::AssocItemContainer::*;
 pub use self::BorrowKind::*;
 pub use self::IntVarValue::*;
@@ -1434,7 +1436,7 @@ fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHas
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ParamEnv<'tcx> {
-    fn try_super_fold_with<F: ty::fold::FallibleTypeFolder<'tcx>>(
+    fn try_fold_with<F: ty::fold::FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
@@ -1445,7 +1447,7 @@ fn try_super_fold_with<F: ty::fold::FallibleTypeFolder<'tcx>>(
         ))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.caller_bounds().visit_with(visitor)?;
         self.reveal().visit_with(visitor)?;
         self.constness().visit_with(visitor)

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1,6 +1,9 @@
 use crate::mir::interpret::{AllocRange, ConstValue, GlobalAlloc, Pointer, Provenance, Scalar};
 use crate::ty::subst::{GenericArg, GenericArgKind, Subst};
-use crate::ty::{self, ConstInt, DefIdTree, ParamConst, ScalarInt, Term, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{
+    self, ConstInt, DefIdTree, ParamConst, ScalarInt, Term, Ty, TyCtxt, TypeFoldable,
+    TypeSuperFoldable,
+};
 use rustc_apfloat::ieee::{Double, Single};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sso::SsoHashSet;

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -4,7 +4,7 @@
 
 use crate::mir::interpret;
 use crate::mir::ProjectionKind;
-use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeVisitor};
+use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use crate::ty::print::{with_no_trimmed_paths, FmtPrinter, Printer};
 use crate::ty::{self, InferConst, Lift, Term, Ty, TyCtxt};
 use rustc_data_structures::functor::IdFunctor;
@@ -672,27 +672,24 @@ fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
 
 /// AdtDefs are basically the same as a DefId.
 impl<'tcx> TypeFoldable<'tcx> for ty::AdtDef<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _folder: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>, U: TypeFoldable<'tcx>> TypeFoldable<'tcx> for (T, U) {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
     ) -> Result<(T, U), F::Error> {
         Ok((self.0.try_fold_with(folder)?, self.1.try_fold_with(folder)?))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.0.visit_with(visitor)?;
         self.1.visit_with(visitor)
     }
@@ -701,7 +698,7 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
 impl<'tcx, A: TypeFoldable<'tcx>, B: TypeFoldable<'tcx>, C: TypeFoldable<'tcx>> TypeFoldable<'tcx>
     for (A, B, C)
 {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
     ) -> Result<(A, B, C), F::Error> {
@@ -712,7 +709,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         ))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.0.visit_with(visitor)?;
         self.1.visit_with(visitor)?;
         self.2.visit_with(visitor)
@@ -734,7 +731,7 @@ impl<'tcx, T, E> TypeFoldable<'tcx> for Result<T, E> {
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Rc<T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
         mut self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
@@ -772,13 +769,13 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         (**self).visit_with(visitor)
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Arc<T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
         mut self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
@@ -816,115 +813,96 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         (**self).visit_with(visitor)
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.try_map_id(|value| value.try_fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         (**self).visit_with(visitor)
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Vec<T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.try_map_id(|t| t.try_fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<[T]> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.try_map_id(|t| t.try_fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::EarlyBinder<T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.try_map_bound(|ty| ty.try_fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.as_ref().0.visit_with(visitor)
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::Binder<'tcx, T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        self.try_map_bound(|ty| ty.try_fold_with(folder))
-    }
-
     fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         folder.try_fold_binder(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        self.as_ref().skip_binder().visit_with(visitor)
-    }
-
     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_binder(self)
     }
 }
 
-impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>> {
+impl<'tcx, T: TypeFoldable<'tcx>> TypeSuperFoldable<'tcx> for ty::Binder<'tcx, T> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_poly_existential_predicates(v))
+        self.try_map_bound(|ty| ty.try_fold_with(folder))
     }
 
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.as_ref().skip_binder().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_poly_existential_predicates(v))
+    }
+
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|p| p.visit_with(visitor))
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ProjectionKind> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         ty::util::fold_list(self, folder, |tcx, v| tcx.intern_projs(v))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ty::instance::Instance<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         use crate::ty::InstanceDef::*;
         Ok(Self {
             substs: self.substs.try_fold_with(folder)?,
@@ -950,7 +928,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         use crate::ty::InstanceDef::*;
         self.substs.visit_with(visitor)?;
         match self.def {
@@ -972,19 +950,26 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
 }
 
 impl<'tcx> TypeFoldable<'tcx> for interpret::GlobalId<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(Self { instance: self.instance.try_fold_with(folder)?, promoted: self.promoted })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.instance.visit_with(visitor)
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_ty(self)
+    }
+
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_ty(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for Ty<'tcx> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
@@ -1029,10 +1014,6 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         Ok(if *self.kind() == kind { self } else { folder.tcx().mk_ty(kind) })
     }
 
-    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
-        folder.try_fold_ty(self)
-    }
-
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match self.kind() {
             ty::RawPtr(ref tm) => tm.visit_with(visitor),
@@ -1074,50 +1055,36 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
             | ty::Foreign(..) => ControlFlow::CONTINUE,
         }
     }
-
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        visitor.visit_ty(*self)
-    }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ty::Region<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        Ok(self)
-    }
-
     fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         folder.try_fold_region(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        ControlFlow::CONTINUE
-    }
-
     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_region(*self)
     }
 }
 
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Region<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        _folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
 impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
     fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         folder.try_fold_predicate(self)
     }
 
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        let new = self.kind().try_fold_with(folder)?;
-        Ok(folder.tcx().reuse_or_mk_predicate(self, new))
-    }
-
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        self.kind().visit_with(visitor)
-    }
-
     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         visitor.visit_predicate(*self)
     }
@@ -1131,33 +1098,51 @@ fn has_type_flags(&self, flags: ty::TypeFlags) -> bool {
     }
 }
 
-impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Predicate<'tcx> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_predicates(v))
+        let new = self.kind().try_fold_with(folder)?;
+        Ok(folder.tcx().reuse_or_mk_predicate(self, new))
     }
 
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.kind().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_predicates(v))
+    }
+
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|p| p.visit_with(visitor))
     }
 }
 
 impl<'tcx, T: TypeFoldable<'tcx>, I: Idx> TypeFoldable<'tcx> for IndexVec<I, T> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         self.try_map_id(|x| x.try_fold_with(folder))
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ty::Const<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_const(self)
+    }
+
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_const(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Const<'tcx> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
@@ -1171,25 +1156,14 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         }
     }
 
-    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
-        folder.try_fold_const(self)
-    }
-
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.ty().visit_with(visitor)?;
         self.val().visit_with(visitor)
     }
-
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        visitor.visit_const(*self)
-    }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(match self {
             ty::ConstKind::Infer(ic) => ty::ConstKind::Infer(ic.try_fold_with(folder)?),
             ty::ConstKind::Param(p) => ty::ConstKind::Param(p.try_fold_with(folder)?),
@@ -1201,7 +1175,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match *self {
             ty::ConstKind::Infer(ic) => ic.visit_with(visitor),
             ty::ConstKind::Param(p) => p.visit_with(visitor),
@@ -1215,14 +1189,11 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
 }
 
 impl<'tcx> TypeFoldable<'tcx> for InferConst<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        _folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _folder: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }
@@ -1232,6 +1203,12 @@ fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Se
         folder.try_fold_unevaluated(self)
     }
 
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_unevaluated(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Unevaluated<'tcx> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,
         folder: &mut F,
@@ -1243,34 +1220,27 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         })
     }
 
-    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        visitor.visit_unevaluated(*self)
-    }
-
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.substs.visit_with(visitor)
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ty::Unevaluated<'tcx, ()> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         Ok(self.expand().try_fold_with(folder)?.shrink())
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.expand().visit_with(visitor)
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for hir::Constness {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
         Ok(self)
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
         ControlFlow::CONTINUE
     }
 }

compiler/rustc_middle/src/ty/sty.rs

@@ -7,7 +7,8 @@
 use crate::ty::subst::{GenericArg, InternalSubsts, Subst, SubstsRef};
 use crate::ty::InferTy::*;
 use crate::ty::{
-    self, AdtDef, DefIdTree, Discr, Term, Ty, TyCtxt, TypeFlags, TypeFoldable, TypeVisitor,
+    self, AdtDef, DefIdTree, Discr, Term, Ty, TyCtxt, TypeFlags, TypeFoldable, TypeSuperFoldable,
+    TypeVisitor,
 };
 use crate::ty::{List, ParamEnv};
 use polonius_engine::Atom;

compiler/rustc_middle/src/ty/subst.rs

@@ -2,7 +2,9 @@
 
 use crate::mir;
 use crate::ty::codec::{TyDecoder, TyEncoder};
-use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeVisitor};
+use crate::ty::fold::{
+    FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitor,
+};
 use crate::ty::sty::{ClosureSubsts, GeneratorSubsts, InlineConstSubsts};
 use crate::ty::{self, Lift, List, ParamConst, Ty, TyCtxt};
 
@@ -196,10 +198,7 @@ fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
 }
 
 impl<'tcx> TypeFoldable<'tcx> for GenericArg<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => lt.try_fold_with(folder).map(Into::into),
             GenericArgKind::Type(ty) => ty.try_fold_with(folder).map(Into::into),
@@ -207,7 +206,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         match self.unpack() {
             GenericArgKind::Lifetime(lt) => lt.visit_with(visitor),
             GenericArgKind::Type(ty) => ty.visit_with(visitor),
@@ -425,10 +424,7 @@ pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> SubstsR
 }
 
 impl<'tcx> TypeFoldable<'tcx> for SubstsRef<'tcx> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         // This code is hot enough that it's worth specializing for the most
         // common length lists, to avoid the overhead of `SmallVec` creation.
         // The match arms are in order of frequency. The 1, 2, and 0 cases are
@@ -454,16 +450,13 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }
 
 impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
         // This code is fairly hot, though not as hot as `SubstsRef`.
         //
         // When compiling stage 2, I get the following results:
@@ -493,7 +486,7 @@ fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         }
     }
 
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))
     }
 }

compiler/rustc_middle/src/ty/util.rs

@@ -1,11 +1,12 @@
 //! Miscellaneous type-system utilities that are too small to deserve their own modules.
 
 use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
-use crate::ty::fold::{FallibleTypeFolder, TypeFolder};
 use crate::ty::layout::IntegerExt;
 use crate::ty::query::TyCtxtAt;
 use crate::ty::subst::{GenericArgKind, Subst, SubstsRef};
-use crate::ty::{self, DefIdTree, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{
+    self, DefIdTree, FallibleTypeFolder, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
+};
 use rustc_apfloat::Float as _;
 use rustc_ast as ast;
 use rustc_attr::{self as attr, SignedInt, UnsignedInt};

compiler/rustc_monomorphize/src/polymorphize.rs

@@ -13,7 +13,7 @@
 };
 use rustc_middle::ty::{
     self,
-    fold::{TypeFoldable, TypeVisitor},
+    fold::{TypeFoldable, TypeSuperFoldable, TypeVisitor},
     query::Providers,
     subst::SubstsRef,
     Const, Ty, TyCtxt,

compiler/rustc_privacy/src/lib.rs

@@ -20,11 +20,10 @@
 use rustc_middle::middle::privacy::{AccessLevel, AccessLevels};
 use rustc_middle::span_bug;
 use rustc_middle::thir::abstract_const::Node as ACNode;
-use rustc_middle::ty::fold::TypeVisitor;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::{self, Const, DefIdTree, GenericParamDefKind};
-use rustc_middle::ty::{TraitRef, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{TraitRef, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use rustc_session::lint;
 use rustc_span::hygiene::Transparency;
 use rustc_span::symbol::{kw, Ident};
@@ -181,7 +180,8 @@ impl<'tcx, V> TypeVisitor<'tcx> for DefIdVisitorSkeleton<'_, 'tcx, V>
 
     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<V::BreakTy> {
         let tcx = self.def_id_visitor.tcx();
-        // InternalSubsts are not visited here because they are visited below in `super_visit_with`.
+        // InternalSubsts are not visited here because they are visited below
+        // in `super_visit_with`.
         match *ty.kind() {
             ty::Adt(ty::AdtDef(Interned(&ty::AdtDefData { did: def_id, .. }, _)), ..)
             | ty::Foreign(def_id)

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -7,7 +7,7 @@
 use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt as _};
 use rustc_infer::traits::{Obligation, ObligationCause, TraitEngine};
-use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts};
 use rustc_middle::ty::{self, OpaqueHiddenType, OpaqueTypeKey, ToPredicate, Ty, TyCtxt};
 use rustc_span::Span;

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -6,7 +6,7 @@
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::InferCtxt;
 use crate::traits::project::ProjectAndUnifyResult;
-use rustc_middle::ty::fold::TypeFolder;
+use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{Region, RegionVid, Term};
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -27,7 +27,7 @@
 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::error::ExpectedFound;
-use rustc_middle::ty::fold::TypeFolder;
+use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{
     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
 };

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -18,7 +18,9 @@
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
-use rustc_middle::ty::{self, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_middle::ty::{Predicate, ToPredicate};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;

compiler/rustc_trait_selection/src/traits/project.rs

@@ -29,7 +29,7 @@
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
 use rustc_middle::traits::select::OverflowError;
-use rustc_middle::ty::fold::{MaxUniverse, TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{MaxUniverse, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, EarlyBinder, Term, ToPredicate, Ty, TyCtxt};
 use rustc_span::symbol::sym;
@@ -514,7 +514,7 @@ fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                             self.selcx.infcx().report_overflow_error(&obligation, true);
                         }
 
-                        let substs = substs.super_fold_with(self);
+                        let substs = substs.fold_with(self);
                         let generic_ty = self.tcx().bound_type_of(def_id);
                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
                         self.depth += 1;
@@ -531,8 +531,7 @@ fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                 // placeholders (see branch below). *Also*, we know that we can
                 // register an obligation to *later* project, since we know
                 // there won't be bound vars there.
-
-                let data = data.super_fold_with(self);
+                let data = data.fold_with(self);
                 let normalized_ty = if self.eager_inference_replacement {
                     normalize_projection_type(
                         self.selcx,
@@ -581,7 +580,7 @@ fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                 let infcx = self.selcx.infcx();
                 let (data, mapped_regions, mapped_types, mapped_consts) =
                     BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
-                let data = data.super_fold_with(self);
+                let data = data.fold_with(self);
                 let normalized_ty = opt_normalize_projection_type(
                     self.selcx,
                     self.param_env,
@@ -671,7 +670,7 @@ pub fn replace_bound_vars<T: TypeFoldable<'tcx>>(
             universe_indices,
         };
 
-        let value = value.super_fold_with(&mut replacer);
+        let value = value.fold_with(&mut replacer);
 
         (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
     }
@@ -794,7 +793,7 @@ pub fn replace_placeholders<T: TypeFoldable<'tcx>>(
             universe_indices,
             current_index: ty::INNERMOST,
         };
-        value.super_fold_with(&mut replacer)
+        value.fold_with(&mut replacer)
     }
 }
 

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -12,7 +12,7 @@
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::traits::Normalized;
 use rustc_middle::mir;
-use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
 
@@ -205,7 +205,7 @@ fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
                     Reveal::UserFacing => ty.try_super_fold_with(self),
 
                     Reveal::All => {
-                        let substs = substs.try_super_fold_with(self)?;
+                        let substs = substs.try_fold_with(self)?;
                         let recursion_limit = self.tcx().recursion_limit();
                         if !recursion_limit.value_within_limit(self.anon_depth) {
                             let obligation = Obligation::with_depth(
@@ -242,7 +242,7 @@ fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
                 // we don't need to replace them with placeholders (see branch below).
 
                 let tcx = self.infcx.tcx;
-                let data = data.try_super_fold_with(self)?;
+                let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
                 // HACK(matthewjasper) `'static` is special-cased in selection,
@@ -281,7 +281,7 @@ fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
                         &mut self.universes,
                         data,
                     );
-                let data = data.try_super_fold_with(self)?;
+                let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
                 // HACK(matthewjasper) `'static` is special-cased in selection,
@@ -334,7 +334,7 @@ fn try_fold_mir_const(
         &mut self,
         constant: mir::ConstantKind<'tcx>,
     ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
-        let constant_kind = match constant {
+        Ok(match constant {
             mir::ConstantKind::Ty(c) => {
                 let const_folded = c.try_fold_with(self)?;
                 match const_folded.val() {
@@ -347,8 +347,6 @@ fn try_fold_mir_const(
                 }
             }
             mir::ConstantKind::Val(_, _) => constant.try_super_fold_with(self)?,
-        };
-
-        Ok(constant_kind)
+        })
     }
 }

compiler/rustc_trait_selection/src/traits/structural_match.rs

@@ -6,7 +6,7 @@
 use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use rustc_span::Span;
 use std::ops::ControlFlow;
 

compiler/rustc_traits/src/chalk/db.rs

@@ -9,7 +9,7 @@
 use rustc_middle::traits::ChalkRustInterner as RustInterner;
 use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
 use rustc_middle::ty::{
-    self, AssocItemContainer, AssocKind, EarlyBinder, Ty, TyCtxt, TypeFoldable,
+    self, AssocItemContainer, AssocKind, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
 };
 
 use rustc_ast::ast;

compiler/rustc_traits/src/chalk/lowering.rs

@@ -33,9 +33,10 @@
 
 use rustc_ast::ast;
 use rustc_middle::traits::{ChalkEnvironmentAndGoal, ChalkRustInterner as RustInterner};
-use rustc_middle::ty::fold::TypeFolder;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
-use rustc_middle::ty::{self, Binder, Region, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, Binder, Region, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_span::def_id::DefId;
 
 use chalk_ir::{FnSig, ForeignDefId};

compiler/rustc_ty_utils/src/instance.rs

@@ -3,7 +3,9 @@
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::traits::CodegenObligationError;
 use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::{self, Binder, Instance, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, Binder, Instance, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_span::{sym, DUMMY_SP};
 use rustc_trait_selection::traits;
 use traits::{translate_substs, Reveal};

compiler/rustc_typeck/src/check/check.rs

@@ -17,11 +17,10 @@
 use rustc_infer::infer::{RegionVariableOrigin, TyCtxtInferExt};
 use rustc_infer::traits::Obligation;
 use rustc_middle::hir::nested_filter;
-use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
-use rustc_middle::ty::{self, ParamEnv, ToPredicate, Ty, TyCtxt};
+use rustc_middle::ty::{self, ParamEnv, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_span::symbol::sym;
 use rustc_span::{self, Span};

compiler/rustc_typeck/src/check/op.rs

@@ -9,8 +9,9 @@
 use rustc_middle::ty::adjustment::{
     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
 };
-use rustc_middle::ty::fold::TypeFolder;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -22,7 +22,7 @@
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
 use rustc_middle::ty::{
     self, AdtKind, EarlyBinder, GenericParamDefKind, ToPredicate, Ty, TyCtxt, TypeFoldable,
-    TypeVisitor,
+    TypeSuperFoldable, TypeVisitor,
 };
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::{sym, Ident, Symbol};

compiler/rustc_typeck/src/check/writeback.rs

@@ -14,7 +14,7 @@
 use rustc_middle::hir::place::Place as HirPlace;
 use rustc_middle::mir::FakeReadCause;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, PointerCast};
-use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{self, ClosureSizeProfileData, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 use rustc_span::Span;

compiler/rustc_typeck/src/coherence/orphan.rs

@@ -9,7 +9,9 @@
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IgnoreRegions;
-use rustc_middle::ty::{self, ImplPolarity, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, ImplPolarity, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_session::lint;
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::Span;
@@ -439,7 +441,7 @@ fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
             }
 
             match t.kind() {
-                ty::Adt(def, substs) if def.is_phantom_data() => substs.super_visit_with(self),
+                ty::Adt(def, substs) if def.is_phantom_data() => substs.visit_with(self),
                 ty::Adt(def, substs) => {
                     // @lcnr: This is the only place where cycles can happen. We avoid this
                     // by only visiting each `DefId` once.

compiler/rustc_typeck/src/collect/type_of.rs

@@ -8,7 +8,7 @@
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IntTypeExt;
-use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt, TypeFoldable, TypeFolder};
+use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 

compiler/rustc_typeck/src/constrained_generic_params.rs

@@ -1,5 +1,5 @@
 use rustc_data_structures::fx::FxHashSet;
-use rustc_middle::ty::fold::{TypeFoldable, TypeVisitor};
+use rustc_middle::ty::fold::{TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::source_map::Span;
 use std::ops::ControlFlow;

src/librustdoc/clean/auto_trait.rs

@@ -1,7 +1,7 @@
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
-use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable};
+use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable, TypeSuperFoldable};
 use rustc_trait_selection::traits::auto_trait::{self, AutoTraitResult};
 
 use std::fmt::Debug;

src/tools/clippy/clippy_lints/src/unit_types/let_unit_value.rs

@@ -7,7 +7,7 @@
 use rustc_hir::{Expr, ExprKind, PatKind, Stmt, StmtKind};
 use rustc_lint::{LateContext, LintContext};
 use rustc_middle::lint::in_external_macro;
-use rustc_middle::ty::{self, Ty, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{self, Ty, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 
 use super::LET_UNIT_VALUE;